Recording and measuring a weak signal represented by, for example, one to several dozen elementary charge carriers presents challenging and acute problems for the designers of modern sensors and transducers for myriad applications in diverse fields of science and technology. In these sensors and transducers, various primary signals (optical, ultrasonic, mechanical, chemical, radiation, etc.) are transformed into elementary charge carriers, such as electrons, holes, or ions, depending on the specific types and versions of the devices being developed for this purpose. Signal charge packets of such elementary charge carriers are amplified and converted to a signal (e.g., to a voltage signal) that is fed into a recording or analyzing device itself and/or as a feedback signal into a controller of the mechanisms or processes that are monitored with the sensors or transducers.
In many applications, such as those relating to laser information and metering devices, recording and image transfer systems, and radiation or particle detecting systems in the physics and nuclear engineering fields, high-speed sensor devices with critical threshold parameters are in an acute demand. Such applications demand sensors capable of detecting and recording of electrical signals that are not only weak (e.g., as few as one or several elementary charge carriers), but also short in duration and/or rapidly varying (i.e., have a large bandwidth). Accordingly, these applications require a sensor capable of amplifying such electrical signals over a wide bandwidth and with a low noise level. It is well known that the first stage of signal amplification primarily determines the basic parameters and characteristics of the sensor device, such as its threshold sensitivity, signal resolution, and response speed (e.g., bandwidth), and presently, generally two paradigms or approaches are being followed in developing sensors having signal amplification characteristics suited for detecting and recording weak electrical signals.
One widespread approach consists in “perfecting” or optimizing traditional analog amplifiers, in which all sets of the input charge carriers are amplified simultaneously. Noise reduction with simultaneous improvement of the threshold sensitivity is achieved primarily by decreasing the geometric size of the amplifier stage of the device. By applying such a scaling approach to charge-coupled device (CCD) video amplifiers, for example, it is possible to attain a threshold sensitivity of several dozen electrons. Such a scaling approach, however, does not solve the problem of recording signals consisting of several electrons.
Another approach to sensing weak electrical signals is using avalanche amplification (multiplication) of signal carriers, which generally is the most sensitive and high-speed method of amplification known in the art. As is well known, avalanche amplification is based on impact ionization arising in a strong electric field, wherein the signal carriers accelerating in an electric field ionize the atoms of the working medium of the amplifier, thus resulting in multiplication (e.g., duplication) of the signal carriers. At a high multiplication factor, however, it is difficult to stabilize the avalanche amplification operating point. Additionally, the internal (excessive) noise level and the response time grow rapidly with increasing multiplication factor. Due to these problems associated with using a large multiplication factor, traditional avalanche photodiodes use a rather low multiplication factor, M, typically less than 103, that does not allow for detecting and recording signals consisting of several electrons in a wide band.
Avalanche multiplication has also been applied to recording individual ionizing particles using a Geiger-Muller counter. A particle entering such a device initiates an avalanche-like process of multiplication of the signal carriers up to a necessary recording level. More recently, this principle has been successfully used for recording single charge carriers in semiconductor avalanche-type photodiodes. This Geiger-Muller principle of amplification, however, does not allow for distinguishing between signals of one and several input charge carriers (i.e., it does not provide high resolution of the number of charge carriers).
It may be appreciated, therefore, that there remains a need for further advancements and improvements in detecting weak signals, and particularly in providing a system and method for high sensitivity and high resolution detection of signals, as well as for such high resolution detection of weak signals with a high bandwidth.